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Clean(er) Combustion - Carbon Limiting Technologies

Following  Copenhagen, many nations will be gearing up to significantly reduce their greenhouse gas emissions over the coming decades.  With approximately one-quarter of worldwide annual GHG emissions coming from electricity and heat generation,  this will mean that – in order for coal and gas to stay in a picture where levels of atmospheric CO2 are stabilized at or below 450 ppm  –the carbon emissions of such plants must somehow be slashed.  

Typically, “clean” coal or gas refers to a reduction of CO2 emissions by at least 90-95%.  Technically speaking, what options are available for reaching or approaching this benchmark?

More Efficient Plant

Since combustion of 1 tonne of coal produces (on average) about 2 tonnes of CO2 (for gas, the figure is about 1:1 on a mass basis), the power producer can save both on fuel costs and on emissions by getting more Kwh per tonne, i.e., by increasing the thermal efficiency of the burn.  

Coal plants vary widely in efficiency.   While the worldwide average efficiency is about 30%,  this  figure accounts for  a wide range, from more than half-century old Pulverised Coal plant running in the low 20s to new super- and ultra-supercritical plant and IGCC units whose efficiencies can exceed 50%.   Simple open-cycle gas plants are usually more efficient than similarly configured coal units, with new-built units running at about 40%.

Building more efficient plant usually takes the form of : raising the boiler temperature to levels  above 374 degrees C (‘supercritical’ boilers);  or  “combining cycles” by using the waste heat from one turbine assembly to drive another turbine.   Combined cycle technology exists for both gasified coal (Integrated Gasification Combined Cycle) and standard gas (Gas Turbine Combined Cycle), and in the future such assemblies could have efficiencies possibly nearing 70%, although this may be approaching thermodynamic limitations.   

While efficient plant will have lower fuel costs, the capital and operations and maintenance costs – from special alloys needed to build supercritical boilers, for instance, or the maintenance needed for specialized turbines – will tend to be much higher.

Carbon Capture and Storage

Beyond increasing efficiency, it is technically possible to abate stack CO2 emissions in order to get a power plant below the 5-10% net-CO2  “clean”  limbo bar.  The aggregate process, involving separation of CO2 from the emissions stream, capture, transport and long-term storage, is called carbon capture and storage (CCS).

While CCS is at this point nowhere applied at a commercial scale to power production, it is expected that higher future carbon prices (the estimates range from $50 to hundreds of dollars per tonne) would make CCS a commercially viable proposition.  In the meantime, subsidization has been ongoing, if anemic, at the national level and some governments – including the UK – are mandating that all future coal build be “CCS ready”.

Current CCS projects are limited almost exclusively to coal, as the carbon emissions of gas are low enough to make CCS for such plant unprofitable under likely near-term carbon price regimes.   Essentially, the capturing at the plant is done in one of three ways – post-combustion, pre-combustion, and oxy-fueled.  In post-combustion CCS, the effluent gas from the coal burn is cleaned of sulfur and nitrogen oxides then passed through a series of catalytic and/or non-catalytic scrubbing agents which chemically trap the CO2, which is then released as the catalytic agent is “regenerated”, compressed, injected into storage tanks or a pipeline , and transported to a storage site.

With oxy-fueling and pre-combustion, the issue of nitrogen oxide contamination is sidestepped by burning the coal in pure oxygen (oxy-fueling) or by gasifying the coal (pre-combustion), which yields carbon monoxide and water, which are then treated to produce heat, hydrogen gas, and carbon dioxide.  In either case, a purer stream of CO2 is produced for separation, compression, injection and eventual storage.

Because CCS involves extra electricity and heat expenditure -  to separate and pump pure oxygen , regenerate catalysts,  run compressors, and so on – it somewhat counters the effects of other clean combustion modes by introducing an efficiency penalty of between 5 and 15 percent.  Aside from expense, there are other major pieces of the CCS puzzle that need to be fitted into place – including the financing and construction of pipeline networks, certification of the long-term safety and security of underground storage sites, assigning of long-term risk and monitoring responsibilities, and obtaining of land and water use permits, where necessary. 

Nevertheless, CCS is the only technology out there right now which can reliably deliver lower than 10% CO2 “clean” fossil fuel emissions.  With  countries  such as China building a new coal plant every week, the US sitting on probably at least a century of reserves, and even the EU starting to look anew at its still-considerable lignite and brown coal reserves, it seems likely that fossil fuels will not go so "quietly into that good night".    If so,  it hopefully can  be insured that, at the very least, fossil fuel stations will be built from here on with at least a firm technical and planning inclination toward CCS build or retrofit.

If you have more questions about carbon limiting technologies, please contact Energy Edge’s Karl Schultz today.



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